Pancreatic DNAse I has been used as a probe of the structure of the individual globin genes in chromatin since transcriptionally active genes in isolated nuclei have been shown to be sensitive to this enzyme. The conformation of globin genes from uninduced and induced mouse erythroleukemia cells is similar, as exposure of nuclei to DNAse I results in destruction of the globin gene sequence from both cell populations. Thus induction of hemoglobin synthesis in mouse erythroleukemia cells is not based on major alterations in chromatin structure. Two steps in globin gene regulation have been defined: 1) the structure of the globin gene in chromatin and 2) the rate of its transcription or metabolism of the primary transcript. In nuclei from sheep fetal erythroid cells, the gamma but not the beta globin genes are sensitive to DNAse I. Preferential accumulation of gamma mRNA is these cells is based on this difference in the conformation of the individual globin genes in chromatin. In nuclei from adult bone marrow erythroid cells, the gamma and beta globin genes are equally sensitive to DNAse I. The gamma gene is in an active conformation despite failure of the gamma mRNA to accumulate in these cells. Regulation in this instance may occur at the level of transcription or mRNA processing.